This application claims priority from European Patent Application Serial No. 0020097.6, filed Mar. 17, 2000.
1. Field of the Invention
The invention relates to a vulcanizable rubber composition comprising a rubber, sulfur or a sulfur donor, a vulcanization accelerator, and a protective agent. It further relates to a vulcanization process comprising mixing and heating said rubber composition, and to an article of manufacture comprising the rubber vulcanizate obtained by said process.
2. Prior Art
Vulcanizing rubber compositions by heating with sulfur or a sulfur donor, a vulcanization accelerator, and a protective agent has been known in the art for many years.
Protective agents are used to protect the rubber vulcanizate against various forms of ageing, fatigue, and ozone. For example, exposure of pneumatic tires to ozone leads to the formation of ozone cracks in particular in the side walls of the tire. A well-known class of protective agents are N,Nxe2x80x2-disubstituted, in particular N-alkyl,Nxe2x80x2-phenyl p-phenylenediamine derivatives. These N,Nxe2x80x2-disubstituted p-phenylenediamine derivatives typically are also referred to as antidegradants, antiozonants or antioxidants. The reader is directed to Hofmann, Rubber Technology Handbook, Hanser Publishers, Munich 1989, pp. 264-277, in particular pp. 269-270. These antidegradants are commercially available inter alia under the trademark Santoflex(copyright) sold by Flexsys. In the rubber industry, the most frequently used antidegradant is N-(1,3-dimethylbutyl)-Nxe2x80x2-phenyl-p-phenylenediamine or 6PPD. It is to be noted that p-phenylenediamine itself and N-phenyl-p-phenylenediamine or 4-aminodiphenylamine (4-ADPA) are protective agents to some extent, but that these compounds are scorchy and therefore not used in practice.
Rubber vulcanizates having acceptable physical properties can be obtained using the conventional sulfur-vulcanization process. Said vulcanizates are used in a wide variety of applications, for example in various portions of tires (e.g. sidewalls as mentioned above) and in other industrial rubber goods such as hoses and belts. However, during the service life of said vulcanizates the protective agent has to migrate to the surface of the vulcanizate in order to provide protection against in particular ozone. The migration rate of protective agents dictates the lifetime of rubber articles. The faster the migration, the better the initial protection. However, fast migration to the surface of the protective agent will have a negative effect on long-term protection, since the antidegradant will be lost due to surface leaching and volatilization. This phenomenon is more pronounced in natural rubbers than in styrene-butadiene rubbers. With respect to migration, the reader is referred to S.-S. Choi, Bull. Korean Chem. Soc., 19 (1998), 1121-1124. Like styrene-butadiene rubbers, filler-reinforced rubbers are less prone to blooming of the protective agent.
Hence, there is a need in the art for a way of controlling the migration rate of the protective agent thereby improving long-term protection of the rubber vulcanizate. Preferably, the other properties of the rubber vulcanizatexe2x80x94such as scorch time, optimum vulcanization time, abrasion resistance, etc.xe2x80x94should remain unchanged or even better, be improved as well. Surprisingly, we have found a solution to this problem.
In one embodiment, the present invention is a vulcanizable rubber composition comprising a rubber, sulfur or a sulfur donor, a vulcanization accelerator, a protective agent and 0.1 phr or more of an N,Nxe2x80x2-disubstituted p-phenylenediamine protective agent present in the form of a salt with an acid, said composition not comprising an elastomeric copolymer having glycidyl groups.
In a second embodiment the present invention is a vulcanization process comprising mixing and heating the above rubber composition at a temperature of 110-220xc2x0 C. for a period of up to 24 hours with the addition of 0.1 phr or more of the salt of an N,Nxe2x80x2-disubstituted p-phenylenediamine protective agent with an acid during mixing of the rubber composition.
In a third embodiment the present invention is an article of manufacture comprising the rubber vulcanizate obtained by the above process.
Other embodiments of the invention encompass details about relative amounts of reactants, coagents, vulcanization accelerators and unvulcanized rubber compositions, all of which are hereinafter disclosed in the following discussion of each of the facets of the present invention.